1. Field of the Invention
The present invention relates to a process for producing a concentric-circular-gradient functional material having a functional layer gradient in the radial direction, and an apparatus therefore.
2. Discussion of the Background
Hydroxyapatite (HAp), which is a kind of calcium phosphate, is similar to the inorganic component of bone in structure, and has an excellent biocompatibility; therefore, sintered bodies of hydroxyapatite are used as artificial bones, artificial dental roots, and other materials for biogenic use.
However, sintered bodies of hydroxyapatite are low in mechanical properties, such as toughness, in order to be used as materials for biogenic use. Accordingly, the sintered bodies of hydroxyapatite are very easily broken, so that the material is unable to be used as a reliable material.
Thus, in order to overcome a drawback of hydroxyapatite material, which is low in mechanical properties, such as toughness, the following method has been hitherto performed: a method of coating the surface of titanium with hydroxyapatite by plasma spray coating or the like, or a method of preparing a composite sintered material made of titanium and hydroxyapatite, or compounding hydroxyapatite with some other different material, thereby improving the mechanical properties.
However, the spray coating has a problem that when hydroxyapatite is sprayed, the crystal thereof is decomposed due to high temperature in the spraying so as to be made amorphous, whereby the biocompatibility peculiar to hydroxyapatite is removed. The spray coating also has a problem that the coating made of hydroxyapatite is easily peeled from titanium.
According to the sintering method, it is difficult to produce a composite sintered material made of titanium and hydroxyapatite since titanium and hydroxyapatite are different from each other in sintering temperature.
However, Japanese Patent Application Publication No. 2000-128651 discloses that the discharge plasma sintering method is used to sinter titanium and hydroxyapatite, titanium and hydroxyapatite are sufficiently sintered without the advance of the decomposition of hydroxyapatite in the coexistence of titanium and hydroxyapatite; thus, suggested is a composite sintered material wherein a mixed layer containing hydroxyapatite and titanium and a titanium layer are arranged in a concentric circular form, and Japanese Patent Application Publication No. 2001-259017 discloses a three-layered composite sintered material for biogenic use having titanium as a central material, calcium phosphate as a peripheral material, and an intermediate layer between the titanium and the calcium phosphate, wherein the composition of the two components is varied in a gradient form in the intermediate layer and the intermediate layer has a relative density of 90% or more.
However, the two-layered or three-layered composite sintered material has a problem that the material is easily cracked by a difference in thermal expansion coefficient which is generated when raw materials for the sintered material are sintered.
Japanese Patent Application Publication No. 2002-309323 discloses a process for producing a gradient functional material, in which a mixed layer having a stepwise or continuously varied ratio of components is arranged as an intermediate layer between inner and outer layers in order to in order to reduce stress due to thermal expansion when a sintered material having different thermal expansion coefficients is produced.
According to this method, however, at the time of producing a concentric-circular-gradient functional material, it is difficult to form an intermediate layer wherein the ratio of components is varied. About a method for forming such an intermediate layer easily, no specific disclosure is found.